In recent years, due to the growing awareness of global environmental conservation, the spread of electric vehicles and plug-in hybrid vehicles is desired. These vehicles are equipped with a direct current battery that supplies power to a motor during traveling. To charge the direct current battery safely with less electric power when charging the direct current battery from a commercial alternating current power supply, a power supply apparatus having high conversion efficiency and including a function of insulating the commercial power supply from the direct current battery is needed.
PTL 1 discloses a resonant charging apparatus that includes an alternating current to direct current (AC-DC) converter and a resonant direct current to direct current (DC-DC) converter and that raises an input voltage of the resonant DC-DC converter as a battery voltage rises to improve the conversion efficiency.